The purpose of this introductory chapter is to provide knowledge to the reader of the major gases that are present in the atmosphere and their role in atmospheric processes and how these affect the quality of the air that we breathe. This includes many physical phenomena including familiarity with the solar spectrum and factors that act on solar energy as it enters the atmosphere; understanding of how the earth’s atmosphere retains and emits thermal energy; understanding factors that affect the earth’s energy balance; knowledge of how temperature changes with height and factors responsible for these changes; familiarity with the different layers or zones of the atmosphere; understanding relationships among density, pressure, and molecular concentrations of atmospheric gases; understand how physical forces affect atmospheric motion; understanding processes that cause air to flow in cyclonic and anticyclonic patterns; knowing differences between air flow patterns in cyclonic (low-pressure) and anticyclonic (high-pressure) systems; understand what jet streams are and their effects on atmospheric phenomena; familiarity with global air circulation patterns and factors that affect them; understanding the nature of water in the atmosphere and the role it plays in atmospheric processes; understanding the impact of global warming potential of greenhouse gases; and understanding how the atmosphere has evolved over the earth’s history and the phenomena responsible for these changes.